Bending light stronger than ever before by accelerating electrons

Device bends light 140 times stronger than previous experiments.

Ordinary materials refract light, making the straw appear bent or broken as in the left image. Some special materials have negative refraction, which bends light in the opposite direction, as depicted on the right.

When it comes to light, certain types of fabricated materials behave in a radically different manner from ordinary materials like water and glass: they have negative refractive indices, so that light will travel in the opposite direction expected in normal materials. These negatively refracting materials can focus light effectively, even when the wavelength of the light is much larger than the device, allowing microscopic control of optical devices.

A new experiment has achieved extraordinarily large negative refractive indices for radio frequency light. Hosang Yoon, Kitty Y. M. Yeung, Vladimir Umansky, and Donhee Ham constructed a special material which, at very cold temperatures, bent light much more strongly than has been accomplished before. In the process, it provides a new understanding of the interaction between light and electrons.

Refraction—the bending of the path of light when it passes from one medium to another—is a familiar phenomenon. If you place a drinking straw into a glass of water, the straw will appear bent, with the part in the water appearing to extend at a steeper angle than the part in air. However, the straw will not "bend" in the opposite direction from its insertion in ordinary materials: if you imagine a vertical line at the point where the straw enters the water, the refracted image will always be on the opposite side of the line from the piece of the straw in air. In negative refraction, however, the straw would appear on the same side of the imaginary vertical line.

The researchers achieved high negative refraction through a specially designed semiconductor device. This consisted of a set of thin strips of aluminum gallium arsenide (AlGaAs) on top of a substrate of gallium arsenide (GaAs). The construction produced a two-dimensional electron gas (2DEG), where the electrons' motion was confined to the interface between the strips and the substrate. The authors pointed out that if only one strip was used, the device didn't exhibit negative refraction. This is because the strips acted in the same way that atoms do in a crystal lattice, forcing the electrons into a set of behaviors—it's just that, in this case, those behaviors are not possible in normal solids. (The device is an example of a metamaterial, since this behavior can't occur naturally).

Ordinary glass has an index of refraction approximately equal to 1.5 for visible light; previous experiments with negative refraction created materials with indices as large as -5. By contrast, the current study obtained an index of refraction of about -700, a dramatically larger effect. As with normal materials, the index depended on the frequency of the light (which is how prisms split visible light into its component colors). In this case, the highest index of refraction was achieved for frequencies around 10GHz, well within the radio portion of the electromagnetic spectrum.

The authors' model to explain the phenomenon is quasi-Newtonian: the light waves accelerate the electrons, which produce a new electromagnetic wave perpendicular to the original one. In combination, these two sets of waves produce the negative refraction. This model is supported by evidence that the device works mainly at very low temperatures (approximately 4 Kelvin—4°C above absolute zero) where the electrons don't disperse, and fails at higher temperatures where thermal effects dominate.

(Arguably, the model is not purely Newtonian, since electrons in materials don't have the same properties as their free cousins: their effective masses are generally different. In fact, the charge carriers in materials like graphene may be effectively massless, even though, as the authors suggested, graphene has similar 2DEG to the metamaterial they used).

Ordinarily radio waves are hard to steer: they are not amenable to lenses (as visible light is) and focusing requires large reflectors, like the ones used in radio telescopes. Using metamaterials to focus radio waves represents a significant advance in the control of light for microscopic devices.

Whoever made that graphic doesn't understand the effects of negative refraction.

The straw on the right would still slant top left to bottom right, just like the straw on the left, it would just be positioned differently (and no, I won't say where, because my own understanding isn't perfect either).

Whoever made that graphic doesn't understand the effects of negative refraction.

The straw on the right would still slant top left to bottom right, just like the straw on the left, it would just be positioned differently (and no, I won't say where, because my own understanding isn't perfect either).

seemed odd to me that where the straw touched glass at the bottom could be that deceptive.

Ordinarily radio waves are hard to steer: they are not amenable to lenses (as visible light is) and focusing requires large reflectors, like the ones used in radio telescopes. Using metamaterials to focus radio waves represents a significant advance in the control of light for microscopic devices.

? No its not. RF is way easier to steer then optical waves. You can use lenses, reflectors, or coherent arrays. The reason you don't see lenses used very much as in visible light is that lenses are annoying/expensive/crappy and you don't have to bother with them for RF.

The difference in the vis is that you have to use reflective/refractive/diffractive elements because we don't have a good way to build phased arrays at optical frequencies.

sorry captain, best we can do is make the klingons think were left handed.

I admittedly just disturbed my cube mates when I laughed out loud.

I did a talk on metamaterials during my undergrad and it was really awesome learning about the notion that, in fact, cloaking technology is closer than we realize. I can say this with certainty because the military would obviously love to get their hands on, quite literally, invisible craft of any kind.

Whoever made that graphic doesn't understand the effects of negative refraction.

The straw on the right would still slant top left to bottom right, just like the straw on the left, it would just be positioned differently (and no, I won't say where, because my own understanding isn't perfect either).

The picture is accurate. Use Snell's law, sin i/ sinr = n (where i = angle of incidence, r = angle of refraction, and n = index of refraction.) so sin r is negative now, thus the straw slants to the left.

I can't tell you how big a result this is. you can basically build a superlens so, so easily with this stuff, which is like AWESOME.

And Ars, it is high time you guys buckle up, and write a 20,000 page introduction to meta-materials.

Bending is a problematic translation for refraction. I think optical fibers are good at bending light, and they do it gradually along a curve. But especially, what about reflection, it can "bend" even more!?

Whoever made that graphic doesn't understand the effects of negative refraction.

The straw on the right would still slant top left to bottom right, just like the straw on the left, it would just be positioned differently (and no, I won't say where, because my own understanding isn't perfect either).

The picture is accurate. Use Snell's law, sin i/ sinr = n (where i = angle of incidence, r = angle of refraction, and n = index of refraction.) so sin r is negative now, thus the straw slants to the left.

I can't tell you how big a result this is. you can basically build a superlens so, so easily with this stuff, which is like AWESOME.

And Ars, it is high time you guys buckle up, and write a 20,000 page introduction to meta-materials.

The angle of the light is not the same thing as the angle of the straw. When viewing a body of water like this, it's more like a translation or a shift. That's why in the original image the straw jumps down, but the angle of the straw remains the same. The index of refraction determines how far and in what direction this jump is, but the straw itself never becomes mirrored like a lensing effect might create.

Edit:As the bottom tip of the straw gets close to the edge, the translation has to get smaller. So if you line up a sheet of paper with your screen, you can see the tip line up with the original direction of the straw. If you have negative refraction, this would still happen, except that the straw would jump to the right instead of to the left, and the angle would be slightly in the opposite direction so that it lines up again.

Bending is a problematic translation for refraction. I think optical fibers are good at bending light, and they do it gradually along a curve. But especially, what about reflection, it can "bend" even more!?

Optical Fibers are bending light by Total internal reflection, different principle at work there.

Whoever made that graphic doesn't understand the effects of negative refraction.

The straw on the right would still slant top left to bottom right, just like the straw on the left, it would just be positioned differently (and no, I won't say where, because my own understanding isn't perfect either).

The picture is accurate. Use Snell's law, sin i/ sinr = n (where i = angle of incidence, r = angle of refraction, and n = index of refraction.) so sin r is negative now, thus the straw slants to the left.

I can't tell you how big a result this is. you can basically build a superlens so, so easily with this stuff, which is like AWESOME.

And Ars, it is high time you guys buckle up, and write a 20,000 page introduction to meta-materials.

The angle of the light is not the same thing as the angle of the straw. When viewing a body of water like this, it's more like a translation or a shift. That's why in the original image the straw jumps down, but the angle of the straw remains the same. The index of refraction determines how far and in what direction this jump is, but the straw itself never becomes mirrored like a lensing effect might create.

Edit:As the bottom tip of the straw gets close to the edge, the translation has to get smaller. So if you line up a sheet of paper with your screen, you can see the tip line up with the original direction of the straw. If you have negative refraction, this would still happen, except that the straw would jump to the right instead of to the left, and the angle would be slightly in the opposite direction so that it lines up again.

Metamaterials will be tricky for clothes manufacturers, not only can they make your ass look bigger (or smaller!), it can make you look "bendy" vs "stiff". Not good/very good depending on the situation.

Quanticles wrote:

The angle of the light is not the same thing as the angle of the straw.

That is true. But if you work the whole problem you have got a funny geometry and a funny intensity, the metamaterial soaks up light. It would take some doing to get familiar with all effects.

That is why a lazy Snell only tells you certain viewing angles (from above, in this case, where the scattering is small). But it does tell you that the image in general will not look like a photoshop composed of mirrored originals.

Incidentally, I didn't see dbngshm's analysis before I posted mine, perhaps it was queued, perhaps I missed it. Didn't mean to barge in like that.

Paywalled, but thanks. At a guess this older work contains a material with a negative refractive index on the order of common positive indexes. The image from this one will be severely distorted in general.

dbngshm wrote:

Torbjörn Larsson, OM wrote:

Bending is a problematic translation for refraction. I think optical fibers are good at bending light, and they do it gradually along a curve. But especially, what about reflection, it can "bend" even more!?

Optical Fibers are bending light by Total internal reflection, different principle at work there.

Old style fibers yes, not gradient fibers that work by refraction. My original comment contained a note on the several mechanisms, but I considered it confusing for my point there which is that "bending" can be gradual.

That is more like the common image, I think, not having english as first language. Is a knick an example of bend or is it not the same as a bend?

My latter example illustrates separately that "bending" can be thought of as happening by several mechanisms besides refraction.

This all becomes very long winded, ephemeral and tangential. My point was that I think we have retained the technical term of refraction because it isn't like we reach out and "bend" light rays with our hands. If it works differently, expect different results at times.

"When it comes to light, certain types of fabricated materials behave in a radically different manner from ordinary materials like water and glass: they have negative refractive indices, so that light will TRAVEL in the opposite direction expected in normal materials."

WRONG!

Light REFRACTS in the opposite direction. It does not TRAVEL in the opposite direction.

The image is just a photoshop. I'd be much more interested in seeing a ray traced rendering. For example I'd like to see what the caustics would really look like.

It's funny you should say that, my PhD was in negative refraction and as part of it I made an example (2D) ray-optics sandpit - http://hacks.philingrey.com/ray/ (note: doesn't work in IE<9). If you play with it a bit you'll get a sense of basic caustics structures and see that much more interesting things can happen if you have a material with -1<n<0.